Effect of chronic morphine treatment on the biosynthesis of agmatine in rat brain and other tissues

Agmatine prevents development of tolerance to anti-nociceptive effect of ethanol in mice

Drug tolerance is directly correlated with drug abuse and physical dependence. The development of tolerance is manifested as the decline in pharmacological responses of drugs following repeated administration of the constant dose. The present study evaluated the effect of agmatine in ethanol-induced anti-nociception and tolerance in the tail-flick assay in mice. In an acute protocol, ethanol (1 and 2 g/kg, i.p. [intraperitoneally]) and agmatine (20 and 40 μg/mouse, i.c.v. [intracerebroventricularly]) produced significant analgesic effects in mice, as was evident from the increased baseline tail-flick latency when tested 20 minutes after their administration. Agmatine in a per se non-effective dose (5 μg/mouse, i.c.v.), L-arginine (40 μg/mouse, i.c.v.), and arcaine (25 μg/mouse, i.c.v.) significantly potentiated the anti-nociceptive effect of ethanol. Blood ethanol analysis showed no significant differences in blood ethanol concentration between ethanol/saline- and ethanol/agmatine-treated mice, suggesting that the effects of agmatine were not due to any possible effects on the pharmacokinetics of ethanol. In a separate study, mice were injected with ethanol (2 g/kg, i.p., 12%) or saline (1 mL/kg, i.p.) once daily for 9 days. On days 1, 3, 5, 7, and 9 of the experiment, they were subjected to the tail-flick test. Agmatine (5-20 μg/mouse, i.c.v.), L-arginine (40 μg/mouse, i.c.v.), arcaine (25 μg/mouse, i.c.v.), aCSF (2 μL/mouse, i.c.v.), or saline (1 mL/kg, i.p.) was administered daily prior to the first daily ethanol or saline injections, and reaction latencies were determined in the tail-flick assay. Injections of agmatine, L-arginine, and arcaine prevented the development of tolerance to ethanol-induced analgesia. Given that agmatine and its endogenous modulation can prevent tolerance to the anti-nociceptive effects of ethanol, these data suggest it as a possible new therapeutic strategy for the treatment of alcohol use disorder and associated complications.

High-level production of the agmatine in engineered Corynebacterium crenatum with the inhibition-releasing arginine decarboxylase

Background

Agmatine is a member of biogenic amines and is an important medicine which is widely used to regulate body balance and neuroprotective effects. At present, the industrial production of agmatine mainly depends on the chemical method, but it is often accompanied by problems including cumbersome processes, harsh reaction conditions, toxic substances production and heavy environmental pollution. Therefore, to tackle the above issues, arginine decarboxylase was overexpressed heterologously and rationally designed in Corynebacterium crenatum to produce agmatine from glucose by one-step fermentation.

Results

In this study, we report the development in the Generally Regarded as Safe (GRAS) l-arginine-overproducing C. crenatum for high-titer agmatine biosynthesis through overexpressing arginine decarboxylase based on metabolic engineering. Then, arginine decarboxylase was mutated to release feedback inhibition and improve catalytic activity. Subsequently, the specific enzyme activity and half-inhibitory concentration of I534D mutant were increased 35.7% and 48.1%, respectively. The agmatine production of the whole-cell bioconversion with AGM3 was increased by 19.3% than the AGM2. Finally, 45.26 g/L agmatine with the yield of 0.31 g/g glucose was achieved by one-step fermentation of the engineered C. crenatum with overexpression of speAI534D.

Conclusions

The engineered C. crenatum strain AGM3 in this work was proved as an efficient microbial cell factory for the industrial fermentative production of agmatine. Based on the insights from this work, further producing other valuable biochemicals derived from l-arginine by Corynebacterium crenatum is feasible.

Inhibitory influence of agmatine in ethanol withdrawal-induced depression in rats: Behavioral and neurochemical evidence

Although ethanol withdrawal depression is one of the prominent reasons for ethanol consumption reinstatement and ethanol dependence, its neurochemical basis is not clearly understood. The present study investigated the role of the agmatinergic system in ethanol withdrawal-induced depression using the forced swim test (FST) in rats. Chronic exposure of animals to ethanol for 21 days and its abrupt withdrawal produced depression-like behavior, as evidenced by increased immobility time in the FST, compared to the pair-fed control animals. The ethanol withdrawal-induced depression was significantly attenuated by agmatine (20-40 μg/rat, i.c.v. [intracerebroventricularly]), moxonidine (50 μg/rat, i.c.v.), 2-BFI (20 μg/rat, i.c.v.), L-arginine (80 μg/rat, i.c.v.), amino-guanidine (25 μg/rat, i.c.v.), and arcaine (50 μg/rat, i.c.v.) by their once-daily administration during the withdrawal phase (Days 21, 22, and 23). The antidepressant effect of agmatine in ethanol-withdrawn rats was potentiated by the imidazoline receptor I₁ agonist moxonidine (25 μg/rat, i.c.v.) and the imidazoline receptor I₂ agonist, 2-BFI (10 μg/rat, i.c.v.) at their sub-effective doses. On the other hand, it was completely blocked by the imidazoline receptor I₁ antagonist, efaroxan (10 μg/rat, i.c.v.) and the imidazoline receptor I₂ antagonist, idazoxan (4 μg/rat, i.c.v.). In addition, agmatine levels were significantly reduced in brain samples of ethanol-withdrawn rats as compared to the pair-fed control animals. In conclusion, the present study suggests the importance of the endogenous agmatinergic system and the imidazoline receptors system in ethanol withdrawal-induced depression. The data project agmatine as a potential therapeutic target for the alcohol withdrawal-induced depression.

Neuroprotective effects of agmatine in antineoplastic drugs induced neurotoxicity: In vitro study

AIMS

The effects of agmatine, an endogenous substance known to have a neuroprotective effect against neurotoxicity has been investigated.

MATERIAL AND METHODS

The primary neuron culture obtained from neonatal rats was exposed to toxicity with paclitaxel and cisplatin and the effect of agmatine on both acute (1 h) and chronic (24 h) exposure was demonstrated by biochemical and molecular analyses. It was demonstrated that the effect of agmatine before and after agmatine was induced by neurotoxicity before agmatine and the effect of agmatine on the formed and occuring toxicities. In addition to the results of cell viability assay, total oxidant capacity and total antioxidant capacity, we have found the opportunity to elaborate on our molecular mechanisms by elaborating our findings with apoptotic and inflammation markers such as caspase 3, kaspase 9 and TNF alpha.

KEY FINDINGS

The results of our study revealed the effect profile of a protective molecule against pathological neural deaths due to neurodegeneration not only in neurotoxicity due to anticancer drugs.

SIGNIFICANCE

In this context, we tried to reverse neurotoxicity due to anticancer drugs by using agmatine the duration (1 and 24 h) and dosage (10-5 M and 10-6 M) determined.

Neuroprotective effect of agmatine (decarboxylated l-arginine) against oxidative stress and neuroinflammation in rotenone model of Parkinson's disease

Parkinson's disease (PD) is the second most common age-related neurodegenerative disease after Alzheimer's disease, characterized by loss of dopaminergic neurons in substantia nigra pars compacta, accompanied by motor and nonmotor symptoms. The neuropathological hallmarks of PD are well reported, but the etiology of the disease is still undefined; several studies assume that oxidative stress, mitochondrial defects, and neuroinflammation play vital roles in the progress of the disease. The current study was established to investigate the neuroprotective effect of agmatine on a rotenone (ROT)-induced experimental model of PD. Adult male Sprague Dawley rats were subcutaneously injected with ROT at a dose of 2 mg/kg body weight for 35 days. Agmatine was injected intraperitoneally at 50 and 100 mg/kg body weight, 1 h prior to ROT administration. ROT-treated rats that received agmatine showed better performance on beam walking and an elevated number of rears within the cylinder test. In addition, agmatine reduced midbrain malondialdehyde as an indication of lipid peroxidation, pro-inflammatory cytokines including tumor necrosis factor alpha and interleukin-1β, and glial fibrillary acidic protein. Moreover, agmatine was responsible for preventing loss of tyrosine hydroxylase-positive neurons. In conclusion, our study showed that agmatine possesses a dose-dependent neuroprotective effect through its antioxidant and anti-inflammatory activities. These findings need further clinical investigations of agmatine as a promising neuroprotective agent for the future treatment of PD.

Agmatine: multifunctional arginine metabolite and magic bullet in clinical neuroscience?

Agmatine, the decarboxylation product of arginine, was largely neglected as an important player in mammalian metabolism until the mid-1990s, when it was re-discovered as an endogenous ligand of imidazoline and α2-adrenergic receptors. Since then, a wide variety of agmatine-mediated effects have been observed, and consequently agmatine has moved from a wallflower existence into the limelight of clinical neuroscience research. Despite this quantum jump in scientific interest, the understanding of the anabolism and catabolism of this amine is still vague. The purification and biochemical characterization of natural mammalian arginine decarboxylase and agmatinase still are open issues. Nevertheless, the agmatinergic system is currently one of the most promising candidates in order to pharmacologically interfere with some major diseases of the central nervous system, which are summarized in the present review. Particularly with respect to major depression, agmatine, its derivatives, and metabolizing enzymes show great promise for the development of an improved treatment of this common disease.

Chronic agmatine treatment prevents behavioral manifestations of nicotine withdrawal in mice

Smoking cessation exhibits an aversive withdrawal syndrome characterized by both increases in somatic signs and affective behaviors including anxiety and depression. In present study, abrupt withdrawal of daily nicotine injections (2mg/kg, s.c., four times daily, for 10 days) significantly increased somatic signs viz. rearing, grooming, jumping, genital licking, leg licking, head shakes with associated depression (increased immobility in forced swim test) as well as anxiety (decreased the number of entries and time spent in open arm in elevated plus maze) in nicotine dependent animals. The peak effect was observed at 24h time point of nicotine withdrawal. Repeated administration of agmatine (40-80µg/mouse, i.c.v.) before the first daily dose of nicotine from day 5 to 10 attenuated the elevated scores of somatic signs and abolished the depression and anxiety like behavior induced by nicotine withdrawal in dependent animals. However, in separate groups, its acute administration 30min before behavior analysis of nicotine withdrawal was ineffective. This result clearly shows the role of agmatine in development of nicotine dependence and its withdrawal. In extension to behavioral experiments, brain agmatine analyses, carried out at 24h time point of nicotine withdrawal demonstrated marked decrease in basal brain agmatine concentration as compared to control animals. Taken together, these data support the role of agmatine as common biological substrate for somatic signs and affective symptoms of nicotine withdrawal. This data may project therapies based on agmatine in anxiety, depression and mood changes associated with tobacco withdrawal.

Agmatine, a metabolite of L-arginine, reverses scopolamine-induced learning and memory impairment in rats

Agmatine (l-amino-4-guanidino-butane), a metabolite of L-arginine through the action of arginine decarboxylase, is a novel neurotransmitter. In the present study, effects of agmatine on cognitive functions have been evaluated by using one trial step-down passive avoidance and three panel runway task. Agmatine (20, 40, 80 mg/kg i.p.) was administered either in the presence or absence of a cholinergic antagonist, scopolamine (1 mg/kg i.p.). Scopolamine significantly impaired learning and memory in both passive avoidance and three panel runway test. Agmatine did not affect emotional learning, working and reference memory but significantly improved scopolamine-induced impairment of learning and memory in a dose dependent manner. Our results indicate that agmatine, as an endogenous substance, may have an important role in modulation of learning and memory functions.

Determination of α(2)-adrenoceptor and imidazoline receptor involvement in augmentation of morphine and oxycodone analgesia by agmatine and BMS182874

Studies have demonstrated that clonidine (α(2)-adrenoceptor and imidazoline receptor agonist) and BMS182874 (endothelin ET(A) receptor antagonist) potentiate morphine and oxycodone analgesia. Agmatine, an endogenous clonidine-like substance, enhances morphine analgesia. However, its effect on oxycodone analgesia and its interaction with endothelin ET(A) receptor antagonists are not known. The present study was performed to determine the effect of agmatine on morphine and oxycodone analgesia and the involvement of α(2)-adrenoceptors, imidazoline receptors, opioid receptors, and endothelin receptors. Antinociception at various time intervals was determined by the tail-flick latency method in mice. Agmatine produced dose-dependent increase in tail-flick latency, while BMS182874 did not produce any change over the 360-min observation period. Agmatine significantly potentiated morphine as well as oxycodone analgesia which was not altered by BMS182874. BMS182874 pretreatment did not increase the analgesic effect produced by agmatine alone. Agmatine-induced potentiation of morphine and oxycodone analgesia was blocked by idazoxan (imidazoline receptor/α(2)-adrenoceptor antagonist) and yohimbine (α(2)-adrenoceptor antagonist). BMS182874-induced potentiation of morphine or oxycodone analgesia was not affected by yohimbine. However, idazoxan blocked BMS182874-induced potentiation of oxycodone but not morphine analgesia. This is the first report demonstrating that agmatine potentiates not only morphine but also oxycodone analgesia in mice. Potentiation of morphine and oxycodone analgesia by agmatine appears to involve α(2)-adrenoceptors, imidazoline receptors, and opioid receptors. In addition, imidazoline receptors may be involved in BMS182874-induced potentiation of oxycodone but not morphine analgesia. It is concluded that agmatine may be used as an adjuvant in opiate analgesia.

Repeated agmatine treatment attenuates nicotine sensitization in mice: modulation by alpha2-adrenoceptors

Agmatine [2-(4-aminobutyl)guanidine] is an endogenous amine proposed as a neurotransmitter/neuromodulator that binds to multiple target receptors in brain. Besides, many central and peripheral functions, agmatine have been implicated in the process of drug addiction. The purpose of the present study was to examine the effects of centrally injected agmatine on nicotine induced locomotor sensitization in Swiss male mice. Our data shows that repeated injections of nicotine (0.4 mg/kg, sc, twice daily for 7 days) gradually increased locomotion during 7 days development period or after 3 days (nicotine) withdrawal phase challenged with nicotine (0.4 mg/kg, sc) on day 11. Mice were pretreated with agmatine (40-80 microg, icv) or agents known to increase endogenous brain agmatine levels [e.g. an agmatine biosynthetic precursor, L-arginine (80 microg, icv), ornithine decarboxylase inhibitor, difluoromethyl-ornithine (50 microg, icv), diamine oxidase inhibitor, aminoguanidine (25 microg, icv) and agmatinase inhibitor, arcaine (50 microg, icv)] 30 min before daily first nicotine injection or during nicotine withdrawal phase. All these treatments attenuated the development as well as incubation of locomotor sensitization to nicotine. Coadministration of agmatine (20 microg, icv) and alpha(2)-adrenoreceptors agonist, clonidine (0.1 microg, icv) evoked synergistic inhibition of nicotine sensitization. Conversely, prior administration of alpha(2)-adrenoceptor antagonist, yohimbine (5mg/kg, ip) or idazoxan (0.4 mg/kg, ip) reversed the inhibitory effect of agmatine on nicotine sensitization. There was no significant difference in activity between mice injected with any of these agents/saline alone and saline/saline groups. These data indicate that agmatine attenuates nicotine induced locomotor sensitization via a mechanism which may involve alpha(2)-adrenergic receptors. Thus, agmatine might have therapeutic implications in the treatment of nicotine addiction and deserve further investigations.

Agmatine and imidazoline receptors: their role in opioid analgesia, tolerance and dependence

Agmatine is an endogenous amine that is synthesized following the decarboxylation of L-arginine by arginine decarboxylase. Agmatine exists in mammalian brain and has been proposed as a neurotransmitter and/or neurotransmodulator. Agmatine binds to several targets and is considered as an endogenous ligand for imidazoline receptors. This review, mainly based on our research work in the past decade, focused on the modulations by agmatine action on imidazoline receptors to opioid analgesia, tolerance and dependence, and its possible neurochemical mechanisms. We went on to propose that agmatine and imidazoline receptors constitute a novel system of modulating opioid functions.

Agmatine transport into spinal nerve terminals is modulated by polyamine analogs

Agmatine (decarboxylated arginine) is an endogenous amine found in the CNS that antagonizes NMDA receptors and inhibits nitric oxide synthase. Intrathecally administered agmatine inhibits hyperalgesia evoked by inflammation, nerve injury and intrathecally administered NMDA. These actions suggest an antiglutamatergic neuromodulatory role for agmatine in the spinal cord. Such a function would require a mechanism of regulated clearance of agmatine such as neuronal or glial uptake. Consistent with this concept, radiolabeled agmatine has been shown to accumulate in synaptosomes, but the mechanism of this transport has not been fully characterized. The present study describes an agmatine uptake system in spinal synaptosomes that appears driven by a polyamine transporter. [(3)H]Agmatine uptake was Ca(2+), energy and temperature dependent. [(3)H]Agmatine transport was not moderated by L-arginine, L-glutamate, glycine, GABA, norepinephrine or serotonin. In contrast, [(3)H]agmatine uptake was concentration dependently inhibited by unlabeled putrescine and by unlabeled spermidine (at significantly higher concentrations). Similarly, [(3)H]putrescine uptake was inhibited in a concentration-dependent manner by unlabeled agmatine and spermidine. The polyamine analogs paraquat and methylglyoxal bis (guanylhydrazone) inhibited, whereas the polyamine transport enhancer difluoromethylornithine increased, [(3)H]agmatine transport. Taken together, these results suggest that agmatine transport into spinal synaptosomes may be governed by a polyamine transport mechanism.

Effect of agmatine on brain l-citrulline production during morphine withdrawal in rats: A microdialysis study in nucleus accumbens

Agmatine, an endogenous nitric oxide (NO) synthase inhibitor and ligand for imidazoline receptors, has been previously shown to prevent morphine dependence in rats. The present study was designed to investigate NO formation in nucleus accumbens core region (NAcc) during naloxone (NL)-precipitated morphine withdrawal in rats treated with agmatine or l-NAME by using intracerebral microdialysis in freely moving rats, through measuring extracellular l-citrulline concentrations, an indirect sign of NO production since equal amounts of l-citrulline and NO are produced from l-arginine. l-Citrulline levels in the NAcc core did not change following administration of agmatine (40 mg/kg i.p.) or l-NAME (100 mg/kg i.p.) in control rats. Both agmatine and l-NAME attenuated withdrawal symptoms of morphine in NL (2 mg/kg i.p.)-precipitated withdrawal. l-Citrulline levels showing the release of NO increased in morphine-dependent rats during NL-precipitated withdrawal. Agmatine and l-NAME treatments significantly suppressed the increase in l-citrulline levels compared to physiological saline-treated rats in this setting. The results suggest that the release of l-citrulline in NAcc may be involved in the processes of morphine withdrawal and agmatine as an endogenous inhibitor of NO synthase may be one of the factors involved in the changes in the physiology and behavioral state during opioid withdrawal and may have pharmacological importance.

Modulation of agmatine on calcium signal in morphine-dependent CHO cells by activation of IRAS, a candidate for imidazoline I1 receptor

The present study investigated the effects of agmatine action on imidazoline I1 receptor antisera-selected protein (IRAS), a candidate for imidazoline I1 receptor, on prolonged morphine-induced adaptations of calcium signal and long-lasting alterations in gene expression to further elucidate the role of IRAS in opioid dependence. Two cell lines, Chinese hamster ovary cells expressing mu opioid receptor alone (CHO-mu) and expressing mu opioid receptor and IRAS together (CHO-mu/IRAS), were used. After chronic treatment with morphine for 48 h, naloxone induced a significant elevation of intracellular calcium concentration ([Ca2+]i) in CHO-mu and CHO-mu/IRAS cells. Agmatine (0.01-3 microM) concentration-dependently inhibited the naloxone-precipitated [Ca2+]i elevation when co-pretreated with morphine in CHO-mu/IRAS, but not in CHO-mu. Efaroxan, an imidazoline I1 receptor-preferential antagonist, completely reversed the effect of agmatine in CHO-mu/IRAS. Agmatine (1-10 microM) administration after chronic morphine exposure for 48 h partially decreased the [Ca2+]i elevation in CHO-mu/IRAS which was entirely antagonized by efaroxan, but not in CHO-mu. In addition, agmatine (1 microM) co-pretreated with morphine attenuated the naloxone-precipitated increases of cAMP-responsive element binding protein and extracellular signal-regulated kinase 1/2 phosphorylations and c-Fos expression in CHO-mu/IRAS. These effects were blocked by efaroxan as well. Taken together, these results indicate that the agmatine-IRAS action system attenuates the up-regulations of Ca2+ signal and its downstream gene expression in morphine-dependent model in vitro, providing additional evidence to support the contribution of IRAS to opioid dependence.

Agmatine potentiates morphine-induced conditioned place preference in mice: modulation by alpha2-adrenoceptors

The effects of agmatine, an endogenous polyamine metabolite formed by decarboxylation of L-arginine, and its combination with morphine on conditioned place preference (CPP) has been investigated in male mice. Our data show that subcutaneous administration of morphine (1-7.5 mg/kg) significantly increases the time spent in the drug-paired compartment in a dose-dependent manner. Intraperitoneal administration of agmatine (1-40 mg/kg) alone does not induce either CPP or conditioned place aversion, while combination of agmatine and subeffective doses of morphine leads to potent rewarding effects. Lower doses of morphine (0.1, 0.05, and 0.01 mg/kg) are able to induce CPP in mice pretreated with agmatine 1, 5, and 10 mg/kg, respectively. Concomitant intraperitoneal administration of UK 14 304 (0.5 mg/kg), a highly selective alpha2-agonist, with per se noneffective dose of morphine (0.5 mg/kg) and also its combination with noneffective doses of agmatine (1 mg/kg) plus morphine (0.05 mg/kg) produces significant CPP. UK 14 304 (0.05, 0.5 mg/kg) alone, or in combination with agmatine (1, 5 mg/kg) have had no effect. We have further investigated the possible involvement of the alpha2-adrenoceptors in the potentiating effect of agmatine on morphine-induced place preference. Selective alpha2-antagonists, yohimbine (0.005 mg/kg) and RX821002 (0.1, 0.5 mg/kg), block the CPP induced by concomitant administration of agmatine (5 mg/kg) and morphine (0.05 mg/kg). Yohimbine (0.001-0.05 mg/kg) or RX821002 (0.05-0.5 mg/kg) alone or in combination with morphine (0.05 mg/kg) or agmatine (5 mg/kg) fail to show any significant place preference or aversion. Our results indicate that pretreatment of animals with agmatine enhances the rewarding properties of morphine via a mechanism which may involve alpha2-adrenergic receptors.

Agmatine: biological role and therapeutic potentials in morphine analgesia and dependence

Agmatine is an amine that is formed by decarboxylation of L-arginine by the enzyme arginine decarboxylase (ADC) and hydrolyzed by the enzyme agmatinase to putrescine. Agmatine binds to several target receptors in the brain and has been proposed as a novel neuromodulator. In animal studies, agmatine potentiated morphine analgesia and reduced dependence/withdrawal. While the exact mechanism is not clear, the interactions with N-methyl-D-aspartate (NMDA) receptors, alpha2-adrenergic receptors, and intracellular cyclic adenosine monophosphate (cAMP) signaling have been proposed as possible targets. Like other monoamine transmitter molecules, agmatine is rapidly metabolized in the periphery and has poor penetration into the brain, which limits the use of agmatine itself as a therapeutic agent. However, the development of agmatinase inhibitors will offer a useful method to increase endogenous agmatine in the brain as a possible therapeutic approach to potentiate morphine analgesia and reduce dependence/withdrawal. This review provides a succinct discussion of the biological role/therapeutic potential of agmatine during morphine exposure/pain modulation, with an extensive amount of literature cited for further details.

Mechanisms involved in the antinociception caused by agmatine in mice

The present study examined the antinociceptive effects of agmatine in chemical behavioural models of pain. Agmatine (1-30 mg/kg), given by i.p. route, 30 min earlier, produced dose-dependent inhibition of acetic acid-induced visceral pain, with mean ID50 value of 5.6 mg/kg. Given orally, 60 min earlier, agmatine (10-300 mg/kg) also produced dose-related inhibition of the visceral pain caused by acetic acid, with mean ID50 value of 147.3 mg/kg. Agmatine (3-100 mg/kg, i.p.) also caused significant and dose-dependent inhibition of capsaicin- and glutamate-induced pain, with mean ID50 values of 43.7 and 19.5 mg/kg, respectively. Moreover, agmatine (1-100 mg/kg, i.p.) caused marked inhibition of both phases of formalin-induced pain, with mean ID50 values for the neurogenic and the inflammatory phases of 13.7 and 5.6 mg/kg, respectively. The antinociception caused by agmatine in the acetic acid test was significantly attenuated by i.p. treatment of mice with L-arginine (precursor of nitric oxide, 600 mg/kg), naloxone (opioid receptor antagonist, 1 mg/kg), p-chlorophenylalanine methyl ester (PCPA, an inhibitor of serotonin synthesis, 100 mg/kg once a day for 4 consecutive days), ketanserin (a 5-HT2A receptor antagonist, 0.3 mg/kg), ondansetron (a 5-HT3 receptor antagonist, 0.5 mg/kg), yohimbine (an alpha2-adrenoceptor antagonist, 0.15 mg/kg) or by efaroxan (an I1 imidazoline/alpha2-adrenoceptor antagonist, 1 mg/kg). In contrast, agmatine antinociception was not affected by i.p. treatment of animals with pindolol (a 5-HT1A/1B receptor antagonist, 1 mg/kg) or idazoxan (an I2 imidazoline/alpha2-adrenoceptor antagonist, 3 mg/kg). Likewise, the antinociception caused by agmatine was not affected by neonatal pre-treatment with capsaicin. Together, these results indicate that agmatine produces dose-related antinociception in several models of chemical pain through mechanisms that involve an interaction with opioid, serotonergic (i.e., through 5-HT2A and 5-HT3 receptors) and nitrergic systems, as well as via an interaction with alpha2-adrenoceptors and imidazoline I1 receptors.

Pharmacodynamics of morphine-induced neuronal nitric oxide production and antinociceptive tolerance development

Elevated nitric oxide (NO) production has been implicated in the development of morphine antinociceptive tolerance. This study was conducted to establish the temporal relationship between morphine-induced increases in neuronal NO and loss of pharmacologic activity. Five groups of rats equipped with microdialysis probes in the jugular vein and hippocampus received an intravenous infusion of saline or morphine (0.3, 1, 2, or 3 mg/kg/h) for 8 h. Morphine concentrations in the blood and hippocampal microdialysate were determined by LC/MS-MS; NO production was quantified with an amperometric sensor implanted in the contralateral hippocampus. Antinociceptive effect was monitored at selected time points during and following infusion by electrical stimulation vocalization. The data were fit with a pharmacokinetic/pharmacodynamic model to obtain parameters governing morphine disposition, stimulation of NO production, antinociception, and antinociceptive tolerance development. An additional three groups of rats were pretreated with l-arginine, the NO precursor (100, 300, or 500 mg/kg/h for 8 h), to elevate NO concentrations prior to morphine infusion. Morphine administration resulted in a dose-dependent increase in NO production; the time course of altered NO production coincided with the development of antinociceptive tolerance. l-arginine pretreatment initially enhanced morphine-induced analgesia early in the morphine infusion. However, this NO-associated increase in opioid response dissipated rapidly due to a dominant NO-induced loss of antinociception. Pharmacodynamic modeling suggested that this latter effect was consistent with a hyperalgesic response. These data define a strong, time-dependent relationship between morphine-induced stimulation of NO production and tolerance development, identify specific NO-induced alterations in nociceptive processing after morphine administration, and indicate that NO is a key mediator of antinociceptive tolerance development.

Imidazoline binding sites and their ligands: an overview of the different chemical structures

Since Bousquet et al. discovered the imidazoline binding sites (IBS) two decades ago, when they realized that the antihypertensive drug clonidine interacts not only with the alpha2-adrenenoceptors (alpha2-AR) but also with a distinct imidazoline preferring binding site, these receptors have been paid a great deal of attention. At least two subtypes, I1 and I2, have been characterised based on their binding affinity for different radioligands, but their structures still remain unknown. The pharmacological profile of these IBSs has been the objective of several and very thorough reviews. However, a medicinal chemistry overview of the different IBS ligands prepared to date has never been attempted. In this study, we attempt to compile all the different chemical structures reported to date as IBS ligands and classify them in function of their chemical structure and binding affinity for the different IBS subtypes. Thus, we comment on the different endogenous IBS ligands known as well as the drugs described to interact with the I1-IBS which have found application as antihypertensive drugs. Then, we review those compounds described in the literature to interact with the I2-IBS, classifying them by their chemical families (imidazolines, guanidines, 2-aminoimidazolines, beta-carbolines). Finally, some conclusions are drawn.

Fentanyl derivatives bearing aliphatic alkaneguanidinium moieties: a new series of hybrid molecules with significant binding affinity for mu-opioid receptors and I2-imidazoline binding sites

A new series of fentanyl derivatives [i.e., N-[1-(2-phenethyl)-4-piperidyl]-N-(guanidinoalkyl)propanamide] bearing aliphatic alkaneguanidinium moieties were prepared. Their affinities for the micro opioid receptors and for the I(2)-imidazoline binding sites (I(2)-IBS) were determined on human post-mortem prefrontal cortex membranes. All of these hybrid compounds had significant and/or very high affinity for both receptors in the nanomolar range, meaning an improvement compared to the prototype N-[1-(2-phenethyl)-4-piperidyl]-N-(guanidinopropyl)propanamide previously reported.

β-Phenylethylamines and the isoquinoline alkaloids

This review covers beta-phenylethylamines and isoquinoline alkaloids derived from them, including further products of oxidation. condensation with formaldehyde and rearrangement, some of which do not contain an isoquinoline system, together with naphthylisoquinoline alkaloids, which have a different biogenetic origin. The occurrence of the alkaloids, with the structures of new bases, together with their reactions, syntheses and biological activities are reported. The literature from July 2002 to June 2003 is reviewed, with 568 references cited.